Use Core ML Quantizer in Llama Export (#4458)

Summary:
This PR is an initial step to add Core ML quantizer in Llama export. We start with "quantize model with XNNPack quantizer then fully delegate to Core ML backend". "Quantize with Core ML quantizer" is under development

This PR does 2 things:
1. Add Core ML quantizer options then use them in Llama export
2. Use different iOS versions for different features: fp16 model can run on iOS 15, while 8a8w quantization requires iOS 17, and 4w quantization requires iOS 18

Pull Request resolved: https://github.com/pytorch/executorch/pull/4458

Reviewed By: kirklandsign

Differential Revision: D60462384

Pulled By: cccclai

fbshipit-source-id: b8698789c06f9f6314f48f567b414d0929353453